Imaging devices with image transform circuitry for improved motion detection

ABSTRACT

An imaging device may include an image sensor that generates frames of image data in response to incident light with an array of image pixels, and processing circuitry that processes the image data. The processing circuitry may include a transformation circuit that applies transforms to subsampled frames of image data that are generated using a subset of the image pixels to produce transform values, and a comparator circuit that compares the transform values. The processing circuitry may determine that motion has occurred between sequential frames if a difference between a first transform value corresponding to a first image frame and a second transform value corresponding to a second image frame exceeds a threshold value. In response to determining that motion has occurred, the image sensor may generate full-frame image data using all of the pixels of the array of image pixels.

BACKGROUND

This relates generally to imaging devices, and more particularly, toimaging devices that include image sensors that capture image data andtransformation circuitry for transforming the image data and improvingmotion detection capabilities.

Imaging devices are commonly included within electronic devices such ascellular telephones, cameras, and computers and include image sensors tocapture images. In a typical arrangement, an image sensor includes anarray of image pixels arranged in pixel rows and pixel columns. Columnreadout circuitry may be coupled to each pixel column for reading outimage signals from the image pixels.

In some imaging devices, it may be desired to activate the image sensorand generate image data in response to detected motion. Motion detectionis often performed using a passive infrared sensor. When the passiveinfrared sensor detects motion, it will wake a processing unit withinthe device to capture an image using the image sensor. However, passiveinfrared sensors are sensitive to heat movement, and any heat movementcan cause the sensors to trigger, even in the absence of motion, causingthe processing unit to trigger and consume energy when an image istaken.

It would therefore be desirable to be able to provide imaging deviceswith improved motion detection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative electronic device having an imagesensor and processing circuitry for capturing images using an array ofimage pixels in accordance with an embodiment.

FIG. 2 is a diagram of an illustrative pixel array and associatedreadout circuitry for reading out image signals from the pixel array inaccordance with an embodiment.

FIG. 3 is a diagram of an illustrative image signal processor having atransformation circuit and a comparator circuit in accordance with anembodiment.

FIG. 4 is a flow chart of illustrative steps of operating theillustrative image sensor, transformation circuit, and comparatorcircuit of FIGS. 2 and 3 to detect motion in accordance with anembodiment.

FIG. 5 is a graph of illustrative peak responses to motion andbrightness changes in a transformed space at various frequencies inaccordance with an embodiment.

FIG. 6 is a chart of illustrative steps of using a discrete cosinetransform on image data generated in sequential image frames to detectmotion in accordance with an embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention relate to imaging devices, and moreparticularly, to imaging devices having transformation circuitry toprovide for improved motion detection. It will be recognized by oneskilled in the art that the present exemplary embodiments may bepracticed without some or all of these specific details. In otherinstances, well known operations have not been described in detail inorder to not unnecessarily obscure the present embodiments.

Imaging systems having digital camera modules are widely used inelectronic devices such as digital cameras, computers, cellulartelephones, and other electronic devices. A digital camera module mayinclude one or more image sensors that gather incoming light to capturean image. Image sensors may include arrays of image pixels. The pixelsin the image sensors may include photosensitive elements such asphotodiodes that convert the incoming light into electric charge. Imagesensors may have any number of pixels (e.g., hundreds or thousands ormore). A typical image sensor may, for example, have hundreds,thousands, or millions of pixels (e.g., megapixels). Image sensors mayinclude control circuitry such as circuitry for operating the imagepixels and readout circuitry for reading out image signals correspondingto the electric charge generated by the photosensitive elements.

The imaging systems may also have processing circuitry. In someapplications, such as in surveillance cameras, cameras configured tocapture action shots (e.g., a camera with a sport detection mode),vehicular cameras, and other imaging devices, it may be desirable forthe processing circuitry to include motion detection circuitry. Inparticular, the motion detection circuitry may apply a transform (suchas a discrete cosine transform) to image data generated by the imagesensor. By comparing values generated by the transform in sequentialimage frames, the processing circuitry may determine whether there hasbeen motion between the image frames.

FIG. 1 is a diagram of an illustrative imaging system such as anelectronic device that uses an image sensor to capture images.Electronic device 10 of FIG. 1 may be a portable electronic device suchas a camera, a cellular telephone, a tablet computer, a webcam, a videocamera, a video surveillance system, a surveillance camera that takesstill images, an automotive imaging system, a video gaming system withimaging capabilities, or any other desired imaging system or device thatcaptures digital image data. Camera module 12 may be used to convertincoming light into digital image data. Camera module 12 may include oneor more lenses 14 and one or more corresponding image sensors 16. Lenses14 may include fixed and/or adjustable lenses and may includemicrolenses formed on an imaging surface of image sensor 16. Duringimage capture operations, light from a scene may be focused onto imagesensor 16 by lenses 14. Image sensor 16 may include circuitry forconverting analog pixel data into corresponding digital image data to beprovided to storage and processing circuitry 18. If desired, cameramodule 12 may be provided with an array of lenses 14 and an array ofcorresponding image sensors 16.

Storage and processing circuitry 18 may include one or more integratedcircuits (e.g., image processing circuits, microprocessors, storagedevices such as random-access memory and non-volatile memory, etc.) andmay be implemented using components that are separate from camera module12 and/or that form part of camera module 12 (e.g., circuits that formpart of an integrated circuit that includes image sensors 16 or anintegrated circuit within module 12 that is associated with imagesensors 16). Image data that has been captured by camera module 12 maybe processed and stored using processing circuitry 18 (e.g., using animage processing engine on processing circuitry 18, using an imagingmode selection engine on processing circuitry 18, etc.). In someembodiments, processing circuitry 18 may include motion detectioncircuitry that can analyze image frame data from image sensor 16 anddetermine whether motion has occurred between sequential image frames.If desired, processing circuitry 18 may activate and/or adjust imagesensor 16 after determining that motion has occurred. Processed imagedata may, if desired, be provided to external equipment (e.g., acomputer, external display, or other device) using wired and/or wirelesscommunications paths coupled to processing circuitry 18.

Although processing circuitry 18 has been shown as separate from cameramodule 12, at least a portion of processing circuitry 18 may be includedwithin camera module 12, if desired. Additionally or alternatively, aportion of processing circuitry 18 may be included within externalequipment, such as an external computer, if desired.

As shown in FIG. 2, image sensor 16 may include a pixel array 20containing image sensor pixels 22 arranged in rows and columns(sometimes referred to herein as image pixels or pixels) and control andprocessing circuitry 24. Array 20 may contain, for example, hundreds orthousands of rows and columns of image sensor pixels 22. Controlcircuitry 24 may be coupled to row control circuitry 26 and imagereadout circuitry 28 (sometimes referred to as column control circuitry,column readout circuitry, readout circuitry, processing circuitry, orcolumn decoder circuitry). Row control circuitry 26 may receive rowaddresses from control circuitry 24 and supply corresponding row controlsignals such as reset, row-select, charge transfer, dual conversiongain, and readout control signals to pixels 22 over row control paths30. These row control signals may be used to enable dual conversion gainoperations within image sensor 16, if desired. One or more conductivelines such as column lines 32 may be coupled to each column of pixels 22in array 20. Column lines 32 may be used for reading out image signalsfrom pixels 22 and for supplying bias signals (e.g., bias currents orbias voltages) to pixels 22. If desired, during pixel readoutoperations, a pixel row in array 20 may be selected using row controlcircuitry 26 and image signals generated by image pixels 22 in thatpixel row can be read out along column lines 32.

Image readout circuitry 28 (sometimes referred to as column readout andcontrol circuitry 28) may receive image signals (e.g., analog pixelvalues generated by pixels 22) over column lines 32. Image readoutcircuitry 28 may include sample-and-hold circuitry for sampling andtemporarily storing image signals read out from array 20, amplifiercircuitry, analog-to-digital conversion (ADC) circuitry, bias circuitry,column memory, latch circuitry for selectively enabling or disabling thecolumn circuitry, or other circuitry that is coupled to one or morecolumns of pixels in array 20 for operating pixels 22 and for readingout image signals from pixels 22. Sample-and-hold circuitry in readoutcircuitry 28 may be used to read out charge generated by image pixels 22using correlated double sampling operations. ADC circuitry in readoutcircuitry 28 may convert analog pixel values received from array 20 intocorresponding digital pixel values (sometimes referred to as digitalimage data or digital pixel data). Image readout circuitry 28 may supplydigital pixel data to control and processing circuitry 24 and/orprocessor 18 (FIG. 1) for pixels in one or more pixel columns.

If desired, image pixels 22 may include one or more photosensitiveregions for generating charge in response to image light. Photosensitiveregions within image pixels 22 may be arranged in rows and columns onarray 20. Pixel array 20 may be provided with a color filter arrayhaving multiple color filter elements which allows a single image sensorto sample light of different colors. As an example, image sensor pixelssuch as the image pixels in array 20 may be provided with a color filterarray which allows a single image sensor to sample red, green, and blue(RGB) light using corresponding red, green, and blue image sensor pixelsarranged in a Bayer mosaic pattern. The Bayer mosaic pattern consists ofa repeating unit cell of two-by-two image pixels, with two green imagepixels diagonally opposite one another and adjacent to a red image pixeldiagonally opposite to a blue image pixel. In another suitable example,the green pixels in a Bayer pattern are replaced by broadband imagepixels having broadband color filter elements (e.g., clear color filterelements, yellow color filter elements, etc.). These examples are merelyillustrative and, in general, color filter elements of any desired colorand in any desired pattern may be formed over any desired number ofimage pixels 22.

Image sensor 16 may be configured to support a global shutter operation(e.g., pixels 22 may be operated in a global shutter mode). For example,the image pixels 22 in array 20 may each include a photodiode, floatingdiffusion region, and local charge storage region. With a global shutterscheme, all of the pixels in the image sensor are reset simultaneously.A charge transfer operation is then used to simultaneously transfer thecharge collected in the photodiode of each image pixel to the associatedcharge storage region. Data from each storage region may then be readout on a per-row basis, for example. However, this is merelyillustrative. In general, any desired read sequence may be used. Forexample, a rolling shutter operation, a pipelined readout operation, anon-pipelined readout operation, or any other desired readout operationmay be utilized.

In some embodiments, image sensor 16 may generate subsampled image data(e.g., image data generated using fewer than all of image pixels 22).For example, image sensor 16 may generate subsampled image data usingonly the green pixels of image pixels 22, only the red pixels of imagepixels 22, only the blue pixels of image pixels 22, or any other desiredsubset of image pixels 22. For example, subsampled image data may begenerated using every other image pixel of image pixels 22. Subsampledimage data may be used by processing circuitry 18 to determine whethermotion has occurred between subsampled image frames. In response todetermining that motion has occurred, image sensor 16 may be activatedto produce full-frame image data (e.g., image data generated using allimage pixels within array 20). However, this is merely illustrative.

As shown in FIG. 3, image signal processor 18 may include transformationcircuit 34 and comparator circuit 36. Other circuitry may be includedwithin image signal processor 18, but is not shown as not to obfuscatethe drawing. Transformation circuit 34 and comparator circuit 36 may beused to perform motion detection using the image data produced by imagesensor 16. In particular, transform circuit 34 may receive image data ofa first image frame (e.g., a first subsampled image frame) from imagesensor 16 and apply a transform to the image data, generating a firsttransform value. Transform circuit 34 may then receive image data of asecond image frame (e.g., a second subsampled image frame) from imagesensor 16 and apply the transform to the image data, generating a secondtransform value. Comparator circuit 36 may then compare the firsttransform value to the second transform value. If the difference betweenthe first and second transform values is greater than a threshold value,processor 18 may determine that motion has occurred between the firstand second image frames and activate/adjust image sensor 16 to, forexample, capture one or more full image frames.

Transform circuit 34 may apply a discrete cosine transform to the imagedata generated by image sensor 16. For example, the discrete cosinetransform may be given by Equation 1,

$\begin{matrix}{{F{\text{(u,v}\text{)}}} = {\left( \frac{2}{N} \right)^{\frac{1}{2}}\left( \frac{2}{M} \right)^{\frac{1}{2}}\Sigma_{i = 0}^{N - 1}\Sigma_{j = 0}^{M - 1}{{\Lambda (i)} \cdot {\Lambda (j)} \cdot {\cos\left\lbrack {\frac{ \cdot u}{2 \cdot N}\left( {{2i} + 1} \right)} \right\rbrack}}{{\cos\left\lbrack {\frac{ \cdot v}{2 \cdot M}\left( {{2j} + 1} \right)} \right\rbrack} \cdot {f\left( {i,j} \right)}}}} & (1)\end{matrix}$

where F(u,v) is the transform value in the frequency domain, N and M arethe x and y dimensions of the image frame generated by image sensor 16,and f(i,j) is the image data generated by image sensor 16. Inparticular, transform circuit 34 may transform the image data acrosseach entire image frame. In other words, transform circuit 34 maytransform each N by M image frame produced by image sensor 16 (e.g., asubsampled image frame) into transform value F.

Using the discrete cosine transform of Equation 1, other cosinetransform, or any other desired transform, transformation circuit 34 maygenerate a transform value for each image frame of data that isgenerated by image sensor 16, which may then be compared to thetransform value of subsequent image frames. This may eliminate the needfor a frame buffer to store a frame of image data (i.e., the transformvalue alone may be stored, rather than the frame of image data), therebyreducing the memory burden on processor 18 and increasing the frame rateof the imaging device. However, this is merely illustrative. A framebuffer may still be used, if desired.

As previously discussed, image sensor 16 may generate subsampled imagedata (e.g., image data generated using fewer than all of the pixels ofarray 20). This subsampled image data may be processed by transformationcircuit 34 to generate the transform values that are compared bycomparator circuit 36. If a difference between the transform values ofsequential image frames is greater than a threshold value, processor 18may determine that motion has occurred and activate/adjust image sensor16 to generate full-frame image data (e.g., image data generated usingall of the pixels of array 20).

Although transform circuit 34 and comparator circuit 36 have been shownto be within processor 18, this is merely illustrative. In general,transform circuit 34 and comparator circuit 36 may be contained withinany desired portion of imaging system 10, whether inside of cameramodule 12 or outside of camera module 12.

A flowchart with illustrative steps to determine whether motion hasoccurred using processor 18 is shown in FIG. 4.

At step 402, image sensor 16 may generate image data of image frame (n)(e.g., a first image frame). As previously discussed, this may besubsampled image data, generated by fewer than all of the pixels ofarray 20. For example, only the green image pixels of array 20 may beused in generating the image data at step 402. However, this is merelyillustrative. Any subset of pixels of pixel array 20 may be used togenerate the image data, or the whole pixel array 20 may be used.However, it may be desirable to subsample the image data (using a subsetof the pixels) to reduce the energy required to generate the image data.

At step 404, transformation circuit 34 may apply a transform to theimage data of image frame (n) to generate a first transform value. Thetransform may be a discrete cosine transform, such as the discretecosine transform of Equation 1, another cosine transform, or any otherdesired transform.

At step 406, image sensor 16 may generate image data of image frame(n+1) (e.g., a second image frame subsequent to the first image frame).As previously discussed, this may be subsampled image data, generated byfewer than all of the pixels of array 20. For example, only the greenimage pixels of array 20 may be used in generating the image data atstep 402. However, this is merely illustrative. Any subset of pixels ofpixel array 20 may be used to generate the image data, or the wholepixel array 20 may be used. However, it may be desirable to subsamplethe image data (using a subset of the pixels) to conserve the energyrequired to generate the image data. Image frame (n+1) may besampled/subsampled using the same pixels as image frame (n), but this ismerely illustrative. Image frame (n+1) may be sampled in any desiredmanner.

At step 408, transformation circuit 34 may apply a transform to theimage data of image frame (n+1) to generate a second transform value.The transform may be a discrete cosine transform, such as the discretecosine transform of Equation 1, another cosine transform, or any otherdesired transform.

At step 410, comparator circuit 36 may compare the first transform valueto the second transform value. If a difference between the firsttransform value and the second transform value is less than a thresholdvalue, processor 18 may determine that motion did not occur betweenimage frame (n) and image frame (n+1), and the process may proceed alongpath 412. In this way, image sensor 16 may continuously produce imageframes (such as subsampled image frames) that may be transformed andcompared to the transform value of the previous image frame to determinewhether motion has occurred. In particular, every image frame may betransformed and compared to the previous image frame, if desired.

If, on the other hand, the difference between the first transform valueand the second transform value is greater than the threshold value,processor 18 may determine that motion did occur between image frame (n)and image frame (n+1), and the process may proceed along path 414 tostep 416.

At step 416, processor 18 may send a control signal to image sensor 16to capture a full image frame (e.g., image data generated by all of thepixels of array 20). In this way, image sensor 16 may produce subsampledimage frames that can be transformed and compared to previous subsampledimage frames by processor 18. Once processor 18 has determined thatmotion has occurred from a difference between the transform values,image sensor 16 may generate full-frame image data. A basis for thisdifference analysis is shown in FIG. 5.

In FIG. 5, an illustrative graph of peak amplitude of the difference intransform values in sequential image frames versus frequency is shown.In particular, the graph provides an illustrative absolute value of thepeak difference between transform values (cosine functions) at eachfrequency (because the transform values are in the frequency domain—seeEquation 1).

Curve 510 shows the peak difference at each frequency when movementoccurs between sequential image frames. As shown by curve 510, there maybe a peak in the transform value difference between approximately 15-30Hz. However, this is merely illustrative. Movement between sequentialimage frames may cause a peak difference in transform values between17-26 Hz, between 10-25 Hz, or any other range of frequencies.

Curve 520 shows the peak response at each frequency when brightnesschanges between sequential image frames. As shown by curve 520, theremay be a peak in the transform value difference between 1-10 Hz.However, this is merely illustrative. In general, however, there is peakin transform value difference at a lower frequency range when there is adifference in brightness than when there is motion. Because of this,when comparing the difference between transform values, comparatorcircuit 36 may disregard the transform values at low frequency values,such as at 1-10 Hz, and instead focus on the transform values between15-30 Hz, between 17-26 Hz, between 10-25 Hz, or other range offrequencies. In this way, a difference between transform values mayindicate motion, instead of a mere change in brightness between imageframes.

A chart of illustrative steps using the discrete cosine transform onimage data of sequential image frames is shown in FIG. 6. As shown inFIG. 6, image data 602 of frame (n) (e.g., a first image frame) may beproduced. This data may be produced by a subset of pixels of imagesensor 16 or all of the pixels of image sensor 16. Image data 602 may betransformed using transformation circuit 34 into discrete cosinetransform value (n) 604 (e.g., a first transform value). However, othertransforms may be used by transformation circuit 34, if desired. Afterdiscrete cosine transform value 604 has been produced, processor 18 maydiscard image data 602, if desired. In this way, the need for a framebuffer may be eliminated, thereby reducing the memory requirements ofmotion detection. However, image data 602 may be stored in a framebuffer, if desired.

After image data 602 has been produced, image data 606 of frame (n+1)(e.g., a second image frame subsequent to the first image frame)) may beproduced. This data may be produced by a subset of pixels of imagesensor 16 or all of the pixels of image sensor 16. It may be desirableto produce image data 606 using the same pixels of image sensor 16 thatgenerate image data 602, for example. However, any subset of imagepixels may be used. Image data 606 may be transformed usingtransformation circuit 34 into discrete cosine transform value (n+1)608. However, other transforms may be used by transformation circuit 34,if desired. After discrete cosine transform value 608 has been produced,processor 18 may discard image data 606, if desired. In this way, theneed for a frame buffer may be eliminated, thereby reducing the memoryrequirements of motion detection. However, image data 606 may be storedin a frame buffer, if desired.

Comparator circuit 36 may then compare discrete cosine transform value604 to discrete cosine transform value 608. As described above inconnection with FIG. 5, comparator circuit 36 may compare the transformvalues between 15-30 Hz, between 17-26 Hz, between 10-25 Hz, or otherdesired range of frequencies. This may allow the processor to determinewhether motion has occurred between the image frame (n) and image frame(n+1), while avoiding false positives due to brightness changes or otherinconsistencies between image frames.

If comparator circuit 36 determines at block 610 that the differencebetween the transform values at a desired frequency or frequencies isbelow a threshold, processor 18 may determine that no motion hasoccurred between the image frames at block 612. The process may thencontinue, and image data from the next image frame (e.g., (n+2)) may betransformed and compared to transform value 608. In this way, theprocessor 18 may continuously compare image frames to determine whethermotion has occurred.

If comparator circuit 36 determines at block 610 that the differencebetween the transform values at a desired frequency or frequencies isabove a threshold, processor 18 may determine that motion has occurredbetween the image frames at block 614. Processor 18 may then instructimage sensor 16 to take a full-frame image (e.g., as opposed to thesubsampled image frames that may be used for motion detection) inresponse to the detected motion.

Although the charts of FIGS. 4 and 6 end at the detection of motion,this is merely illustrative. In some embodiments, transform circuit 34may transform the full-frame image data generated after detectingmotion, and image sensor 16 may produce full-frame image data untilmotion is no longer detected (e.g., until a difference in transformvalues between full-frame image data is less than a threshold). At thatpoint, image sensor 16 may proceed to produce subsampled image datauntil motion is again detected. However, imaging system 10 may beoperated in any desired manner to detect motion and take appropriateaction.

Various embodiments have been described illustrating imaging deviceshaving transform and comparator circuitry to provide improved motiondetection.

In various embodiments of the present invention, an imaging device thatgenerates images in response to incident light may include an imagesensor having an array of pixels that generate frames of image data inresponse to the incident light, and processing circuitry that processesthe frames of image data. The processing circuitry may include atransformation circuit that transforms each frame of image data toproduce a respective transform value, and a comparator circuit thatcompares each respective transform value to a transform valuecorresponding to a previous frame of image data.

In accordance with some embodiments, the transformation circuit mayapply a discrete cosine transform to each frame of image data to producethe respective transform value.

In accordance with some embodiments, the image sensor may generate theframes of image data using a subset of pixels of the array of pixels.

In accordance with some embodiments, the imaging device may generate theframes of image data using only green pixels of the array of pixels.

In accordance with some embodiments, the processing circuitry may detectthat motion has occurred between a first frame of image data having afirst transform value and a second frame of image data having a secondtransform value in response to the comparator circuit determining that adifference between the first transform value and the second transformvalue exceeds a threshold value.

In accordance with some embodiments, the image sensor may produce afull-frame of image data using all of the pixels of the array of pixelsin response to the processing circuitry determining that motion hasoccurred.

In accordance with some embodiments, the comparator circuit may comparethe respective transform value to the transform value corresponding tothe previous frame of image data (e.g., the first transform value to thesecond transform value) within a frequency range of 15-30 Hz.

In accordance with some embodiments, the imaging device may furtherinclude a camera module, and the image sensor, the transformationcircuit, and the comparator circuit may be within the camera module.

In accordance with some embodiments, the imaging device may furtherinclude a camera module, the image sensor may be within the cameramodule, and the processing circuitry may be separate from the cameramodule.

In accordance with various embodiments, a method of operating an imagingdevice may include generating a first frame of image data with an imagesensor, applying a transform to the first frame of image data with atransformation circuit to generate a first transform value, generating asecond frame of image data with the image sensor, applying the transformto the second frame of image data with the transformation circuit togenerate a second transform value, comparing the first transform valueto the second transform value, and in response to determining that adifference between the first transform value and the second transformvalue exceeds a threshold value, generating full-frame image data withthe image sensor.

In accordance with some embodiments, generating the first frame of imagedata and generating the second frame of image data may includegenerating a first subsampled frame of image data and generating asecond subsampled frame of image data using a subset of image pixels ofan array of image pixels in the image sensor.

In accordance with some embodiments, generating the first subsampledframe of image data and generating the second subsampled frame of imagedata using the subset of image pixels may include generating the firstand second subsampled frames of image data using only green image pixelsof the array of image pixels.

In accordance with some embodiments, generating the full-frame imagedata may include generating the full-frame image data using all of theimage pixels of the array of image pixels in the image sensor.

In accordance with some embodiments, the method may further includeafter applying the transform to the first frame of image data togenerate the first transform value, discarding the first frame of imagedata, and after applying the transform to the second frame of image datato generate the second transform value, discarding the second frame ofimage data.

In accordance with some embodiments, applying the transform to the firstframe of image data and applying the transform to the second frame ofimage data may include applying a discrete cosine transform to the firstframe of image data and to the second frame of image data.

In accordance with some embodiments, comparing the first transform valueto the second transform value may include determining the differencebetween the first transform value and the second transform value withina frequency range of 15-30 Hz.

In accordance with some embodiments, the method may further include inresponse to determining that the difference between the first transformvalue and the second transform value does not exceed the thresholdvalue, applying the transform to a third frame of image data with thetransformation circuit to generate a third transform value, andcomparing the third transform value to the second transform value.

In accordance with various embodiments, an imaging system may include animage sensor that includes an array of pixels. The image sensors sensormay generate frames of subsampled image data using a portion of thearray of pixels and the image sensor may generate full-frame image datausing the entire array of pixels. The imaging system may also include atransformation circuit that may transform the frames of subsampled imagedata to generate transform values, and a comparator circuit that maycompare a first transform value corresponding to a first frame ofsubsampled image data to a second transform value corresponding to asecond frame of subsampled image data. The image sensor may generate thefull-frame image data in response to a difference between the firsttransform value and the second transform value exceeding a thresholdvalue.

In accordance with some embodiments, the image sensor may generate theframes of subsampled image data using only green pixels of the array ofpixels.

In accordance with some embodiments, processing circuitry may determinethat motion has occurred between the first frame of subsampled imagedata and the second frame of subsampled image data based on thedifference between the first transform value and the second transformvalue exceeding the threshold value.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. An imaging device configured to generate images in response to incident light, the imaging device comprising: an image sensor having an array of pixels that generate frames of image data in response to the incident light; and processing circuitry that processes the frames of image data, the processing circuitry comprising: a transformation circuit that transforms each frame of image data to produce a respective transform value, and a comparator circuit that compares each respective transform value to a transform value corresponding to a previous frame of image data.
 2. The imaging device defined in claim 1 wherein the transformation circuit is configured to apply a discrete cosine transform to each frame of image data to produce the respective transform value.
 3. The imaging device defined in claim 2 wherein the image sensor generates the frames of image data using a subset of pixels of the array of pixels.
 4. The imaging device defined in claim 3 wherein the imaging device generates the frames of image data using only green pixels of the array of pixels.
 5. The imaging device defined in claim 4 wherein the processing circuitry is configured to detect that motion has occurred between a first frame of image data having a first transform value and a second frame of image data having a second transform value in response to the comparator circuit determining that a difference between the first transform value and the second transform value exceeds a threshold value.
 6. The imaging device defined in claim 5 wherein the image sensor is configured to produce a full frame of image data using all of the pixels of the array of pixels in response to the processing circuitry determining that motion has occurred.
 7. The imaging device defined in claim 6 wherein the comparator circuit is configured to compare the first transform value to the second transform value within a frequency range of 15-30 Hz.
 8. The imaging device defined in claim 1 further comprising: a camera module, wherein the image sensor, the transformation circuit, and the comparator circuit are within the camera module.
 9. The imaging device defined in claim 1 further comprising: a camera module, wherein the image sensor is within the camera module and wherein the transformation circuit and the comparator circuit are separate from the camera module.
 10. A method of operating an imaging device, the method comprising: generating a first frame of image data with an image sensor; applying a transform to the first frame of image data with a transformation circuit to generate a first transform value; generating a second frame of image data with the image sensor; applying the transform to the second frame of image data with the transformation circuit to generate a second transform value; comparing the first transform value to the second transform value; and in response to determining that a difference between the first transform value and the second transform value exceeds a threshold value, generating full-frame image data with the image sensor.
 11. The method defined in claim 10 wherein generating the first frame of image data and generating the second frame of image data respectively comprise generating a first subsampled frame of image data and generating a second subsampled frame of image data using a subset of image pixels of an array of image pixels in the image sensor.
 12. The method defined in claim 11 wherein generating the first subsampled frame of image data and generating the second subsampled frame of image data using the subset of image pixels comprises generating the first and second subsampled frames of image data using only green image pixels of the array of image pixels.
 13. The method defined in claim 11 wherein generating the full-frame image data comprises generating the full-frame image data using all of the image pixels of the array of image pixels in the image sensor.
 14. The method defined claim 13 further comprising: after applying the transform to the first frame of image data to generate the first transform value, discarding the first frame of image data; and after applying the transform to the second frame of image data to generate the second transform value, discarding the second frame of image data.
 15. The method defined in claim 10 wherein applying the transform to the first frame of image data and applying the transform to the second frame of image data respectively comprise applying a discrete cosine transform to the first frame of image data and to the second frame of image data.
 16. The method defined in claim 15 wherein comparing the first transform value to the second transform value comprises determining the difference between the first transform value and the second transform value within a frequency range of 15-30 Hz.
 17. The method defined in claim 10 further comprising: in response to determining that the difference between the first transform value and the second transform value does not exceed the threshold value: applying the transform to a third frame of image data with the transformation circuit to generate a third transform value, and comparing the third transform value to the second transform value.
 18. An imaging system comprising: an image sensor comprising an array of pixels, wherein the image sensor is configured to generate frames of subsampled image data using a portion of the array of pixels and wherein the image sensor is configured to generate full-frame image data using the entire array of pixels; a transformation circuit that transforms the frames of subsampled image data to generate transform values; and a comparator circuit that compares a first transform value corresponding to a first frame of subsampled image data to a second transform value corresponding to a second frame of subsampled image data, wherein the image sensor is configured to generate the full-frame image data in response to a difference between the first transform value and the second transform value exceeding a threshold value.
 19. The imaging system defined in claim 18 wherein the image sensor is configured to generate the frames of subsampled image data using only green pixels of the array of pixels.
 20. The imaging system defined in claim 18 wherein processing circuitry is configured to determine that motion has occurred between the first frame of subsampled image data and the second frame of subsampled image data based on the difference between the first transform value and the second transform value exceeding the threshold value 